This invention relates to stabilized, melt-processable ethylene/chlorotrifluoroethylene copolymer compositions, particularly to equimolar ethylene/chlorotrifluoroethylene copolymers, and more particularly, to a process for stabilizing equimolar ethylene/chlorotrifluoroethylene copolymers against degradation during fabrication and use at elevated temperatures.
Equimolar ethylene/chlorotrifluoroethylene copolymers exhibit outstanding mechanical, electrical and chemical properties at high temperatures. For example, equimolar ethylene/chlorotrifluoroethylene copolymers resist attack of most organic solvents at ambient as well as elevated temperatures, being only slightly soluble at 100.degree.-150.degree. C. in 2,5-dichlorobenzotrifluoride/benzonitrile or O-dichloro-benzene mixtures of 10:90 to 50:50 volume ratio. They are insoluble in bases and acids, including fuming nitric acid. These copolymers also possess high tensile strength and have melting points above about 200.degree. C. and as high as about 260.degree. C., which melting points are higher than those of the homopolymers of either ethylene as high as 130.degree. C.) or chlorotrifluoroethylene (as high as 215.degree. C.). Equimolar ethylene/chlorotrifluoroethylene copolymers also have outstanding electrical properties. For all of these advantageous properties, they are suitable for making useful articles, such as valves, gaskets, pipes, wire insulation, sheets or films for use in applications where their excellent mechanical, electrical and chemical properties can be used to advantage.
Equimolar ethylene/chlorotrifluoroethylene copolymers, due to their high melting points in excess of about 220.degree. C., require melt fabrication temperatures of above about 250.degree. C., usually within the range of between about 260.degree. C. to 320.degree. C. At these high temperatures, rapid degradation of the polymer occurs resulting in discoloration and loss of chemical, mechanical and electrical properties. In particular, degradation of these properties occurs during extrusion and injection molding operations.
U.S. Pat. No. 3,745,145 discloses equimolar ethylene/chlorotrifluoroethylene copolymer compositions which are stabilized against thermal degradation. The stabilizing system comprises: a phosphite of an organic polyhydric phenol; a salt of a carboxylic acid and a metal of Group II of the Periodic Table; and a thiodipropionic acid ester or alkali metal salt. This stabilizing system is satisfactory for most melt fabricating techniques used to process ethylene/chlorotrifluoroethylene copolymer compositions. There are various melt-fabricating processes, such as transfer molding and injection molding of very large parts, where the temperature and residence time of the process are more severe than encountered in most melt fabricating techniques. Therefore, it is desirable to have an improved stabilizing system for ethylene/chlorotrifluoroethylene composition to be processed at higher temperatures and for longer processing times than normally encountered.
U.S. Pat. Nos.: 3,644,482, 3,285,855, 3,255,136 and 3,244,650 are of interest.